Ch Frequency Response (Custom OFDM)
The trace for Custom OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. modulation is the channel frequency response for the specified spatial stream (MIMO Multiple Input, Multiple Output: A physical layer (PHY) configuration in which both transmitter and receiver use multiple antennas.) and measurement hardware input channel X (where X is the number of the channel).
ChX Channel Frequency Response is computed as the reciprocal of the ChX Equalizer Frequency Response. The Channel Frequency Response provides one complex value for each subcarrier plus an interpolated value for the middle unused Null subcarrier. The equalizer frequency response is normally estimated from the Channel Estimation Sequence (HT high throughput-LTF Long Training field) portion of the OFDM preamble. When the "Equalization Training" parameter is set to Preamble & Data, the equalizer frequency response is estimated using both the Channel Estimation Sequence (HT-LTF) and the payload Data.
The frequency response data results are similar to the Digital Demodulation Eq Impulse Response trace data. The differences are:
- Digital Demodulation computes frequency response by comparing the IQ Meas Time and IQ Ref Time data, while Custom OFDM demodulation computes it from the preamble/data of the burst.
- Digital Demodulation typically uses a running average to average multiple scans when computing the equalizer frequency response. Custom OFDM demodulation does not average, but computes a new equalizer response for each burst.
- The equalization filter is inherent to Custom OFDM Demodulation, it cannot be enabled/disabled like in Digital Demodulation.
Understanding MIMO Trace Data Results
There is a channel frequency response trace data result for each input channel and spatial stream (Ch X Strm X) path for the measurement MIMO system.
For example, a 2x2 MIMO system is comprised of 2 input channels and 2 spatial streams and therefore has four channel frequency response trace data results, one for each input-channel/spatial-stream path; 1) Ch1 Strm1, 2) Ch1 Strm 2, 3) Ch2 Strm 1, and 4) Ch 2 Strm 2 (where "Ch" is the VSA measurement hardware input channel and "Strm" is the spatial stream). The following graphic shows the trace data naming convention for multi-channel 2x2 MIMO system.
In general, the VSA only supports MIMO configurations with 1 data stream per input channel. However, there are a few exceptions, see 802.11n/ac book, 802.11n/ac Measurement Capabilities topic for more information.
The following table shows how the number of frequency response measurements expand as the number of channel inputs and spatial streams increase:
| Number of Input Channels | Channel Frequency Response Measurements | ||
| 1 | Stream1 Ch1 | ||
| 2 | Stream1 Ch1 | Stream2 Ch1 | |
| Stream1 Ch2 | Stream2 Ch2 | ||
| 3 | Stream1 Ch1 | Stream2 Ch1 | Stream3 Ch1 |
| Stream1 Ch2 | Stream2 Ch2 | Stream3 Ch2 | |
| Stream1 Ch3 | Stream2 Ch3 | Stream3 Ch3 | |
See Also
